An electrically resettable railway brake, for instance one of the type described in U.S. Pat. No. 4,175,645, has an actuating member connected with a caliper arm carrying a brakeshoe and displaceable in a forward stroke, i.e. from a disengaged or rest position into an engaged or working position, by a spring driving the brakeshoe against a rotary element to be arrested. The release of the brake, i.e. the reopening of the jaws of the caliper, is performed by the energization of an electric motor whose electromotive force opposes and overcomes the force of the spring assembly. If current should fail, the spring force immediately recloses the brake which therefore acts as a so-called dead man's safety device.
The brakeshoes carried at the free ends of the caliper jaws have linings subject to inevitable wear. Means for compensating that wear are known, for example, from German Pat. No. 2,512,786 and from Swiss Pat. No. 612,134 according to which a ratchet mechanism automatically resets the retracted or rest position of the actuating member whenever the closure stroke of the brake exceeds a predetermined distance. Since the spring assembly is under a maximum stress in the retracted position of the actuating member, the progressive advance of the closure stroke with continuing wear of the brake linings is accompanied by a proportional relaxation of the spring force so that, in the working position of the brake, the rotary element is less firmly gripped between the jaws when the linings are partly worn.
With a different type of electromechanically reciprocable load, e.g. a door or gate to be closed by a spring force and to be reopened by a countervailing electromotive force, the spring force progressively weakens during the closure stroke. Thus, the force exerted at the beginning of that stroke must substantially exceed that required to keep the door closed. This means that a person or an object accidentally standing in the doorway at the beginning of closure could be subjected to considerable pressure between the door and its frame even though only a fraction of that pressure is available in the terminal phase of the closing motion.